lkml.org 
[lkml]   [2024]   [Apr]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [RFC PATCH 1/5] mm: memory: extend finish_fault() to support large folio
From


On 2024/4/23 19:03, Ryan Roberts wrote:
> On 22/04/2024 08:02, Baolin Wang wrote:
>> Add large folio mapping establishment support for finish_fault() as a preparation,
>> to support multi-size THP allocation of anonymous shared pages in the following
>> patches.
>>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>> mm/memory.c | 25 ++++++++++++++++++-------
>> 1 file changed, 18 insertions(+), 7 deletions(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index b6fa5146b260..094a76730776 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -4766,7 +4766,10 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>> {
>> struct vm_area_struct *vma = vmf->vma;
>> struct page *page;
>> + struct folio *folio;
>> vm_fault_t ret;
>> + int nr_pages, i;
>> + unsigned long addr;
>>
>> /* Did we COW the page? */
>> if ((vmf->flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED))
>> @@ -4797,22 +4800,30 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>> return VM_FAULT_OOM;
>> }
>>
>> + folio = page_folio(page);
>> + nr_pages = folio_nr_pages(folio);
>> + addr = ALIGN_DOWN(vmf->address, nr_pages * PAGE_SIZE);
>
> I'm not sure this is safe. IIUC, finish_fault() is called for any file-backed
> mapping. So you could have a situation where part of a (regular) file is mapped
> in the process, faults and hits in the pagecache. But the folio returned by the
> pagecache is bigger than the portion that the process has mapped. So you now end
> up mapping beyond the VMA limits? In the pagecache case, you also can't assume
> that the folio is naturally aligned in virtual address space.

Good point. Yes, I think you are right, I need consider the VMA limits,
and I should refer to the calculations of the start pte and end pte in
do_fault_around().

>> vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
>> - vmf->address, &vmf->ptl);
>> + addr, &vmf->ptl);
>> if (!vmf->pte)
>> return VM_FAULT_NOPAGE;
>>
>> /* Re-check under ptl */
>> - if (likely(!vmf_pte_changed(vmf))) {
>> - struct folio *folio = page_folio(page);
>> -
>> - set_pte_range(vmf, folio, page, 1, vmf->address);
>> - ret = 0;
>> - } else {
>> + if (nr_pages == 1 && vmf_pte_changed(vmf)) {
>> update_mmu_tlb(vma, vmf->address, vmf->pte);
>> ret = VM_FAULT_NOPAGE;
>> + goto unlock;
>> + } else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
>
> I think you have grabbed this from do_anonymous_page()? But I'm not sure it
> works in the same way here as it does there. For the anon case, if userfaultfd
> is armed, alloc_anon_folio() will only ever allocate order-0. So we end up in

IMO, the userfaultfd validation should do in the vma->vm_ops->fault()
callback, to make sure the nr_pages is always 1 if userfaultfd is armed.

> the vmf_pte_changed() path, which will allow overwriting a uffd entry. But here,
> there is nothing stopping nr_pages being greater than 1 when there could be a
> uffd entry present, and you will fail due to the pte_range_none() check. (see
> pte_marker_handle_uffd_wp()).

So if we do the userfaultfd validation in ->fault() callback, then here
we can use the same logic as with anonymous case.

\
 
 \ /
  Last update: 2024-04-24 05:24    [W:0.133 / U:0.448 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site